A nail in somebody’s/something’s coffin
Definition 1 of 1
Pro Tip 1/3Metaphorical Meaning
It's used to represent gradual harm, not actual nails or coffins. 
Skipping classes was another nail in his academic coffin.
Pro Tip 2/3Negative Context
The phrase usually describes negative developments leading to failure. 
The new tax laws became a nail in the coffin for the struggling small business.
Pro Tip 3/3Not Literal
Understand it's an idiom; don't take the words 'nail' or 'coffin' literally. 
Mismanagement was a nail in the coffin of the project.